Area 51 is a restricted site in Nevada with an almost cult-like mythology surrounding it. Some people claim it's a standard military operation site, but others swear that it within its gated walls exists proof about extraterrestrial life.
Before we get into public knowledge, I want to throw in my thoughts on this. I was an intelligence officer in the Air Force and I maybe shouldn't post this on the internet but my final assignment was in a place that rhymes with Rational Maturity Agency, and while the government definitely does some cool classified work there, I can say with high confidence that no one would be able to keep aliens a secret. At least not the kinds of aliens we tell stories about. Maybe Area 51 has some petri dishes of extraterrestrial amoebas...but I really doubt it.
What's more likely is that the government tests advanced technology out in the desert. The flight pattern of an aircraft like the Harrier could totally be interpreted as a UFO at night.
As the video below states, "No doubt aircraft are still being secretly built and tested there today."
You can check declassified documents to learn about what has been tested on site in the past. In fact, because of the Freedom Of Information Act, U.S. citizens have the right to request access to federal agency records; there are limitations, of course, but it's a fun pastime to ask the "Rational Maturity Agency" for documents concerning things like aliens or Elvis or other conspiracies.
Check out this fascinating video to get a no-nonsense re-cap of what we know about Area 51 — then let me know what you think is going on out there!